Current techniques for back to front alignment generally fall into two categories, i.e., Moire and dual grating techniques. Moire techniques rely on the generation of Moire fringes from two misaligned patterns, while dual grating techniques rely on replicating identical gratings on the front and back surfaces, measuring the relative strengths of the twice diffracted orders thereof and then calculating the misalignment of the gratings based thereon.
Moire techniques often do not work satisfactorily with relatively thick substrates which are commonly used in many optical applications and cannot produce quantitative data as accurately as desired in many applications. Dual grating techniques are relatively sensitive to process variations, e.g., variations in the substrate thickness, or variations in replicating the gratings, and require relatively complicated and expensive implementations.
It is desirable to devise a technique for providing accurate back to front alignment which can be used with relatively thick substrates and which is not only less sensitive to processing variations but is also relatively simple and inexpensive to implement.